1. Field of the Invention
The present invention relates to a switched capacitor resonator and a sigma-delta modulator using the same, and more particularly, to a switched capacitor resonator designed to operate at high speed by reducing a settling time constant of an integrator in the switched capacitor resonator having the integrator configured with an operational amplifier, and a sigma-delta modulator using the switched capacitor resonator.
2. Description of the Related Art
Recently, in line with rapid development of wireless communication technology, studies are being conducted on a variety of new communication systems and circuits, and there is also an ongoing demand for a fusion type communication system architecture incorporating a traditional wireless communication system and a new wireless communication system. To realize such a fusion type communication system architecture, it is essentially required to digitalize a wireless communication system. For this reason, it is being considered that a digital-to-analog converter (ADC) becomes more and more important in digitalization.
The ADC is an important circuitry configured to convert an analog signal to a digital signal. Examples of the ADC may typically include a flash ADC, a pipeline ADC, a successive approximation (SAR) ADC, and a sigma-delta ADC. In particular, the sigma-delta ADC is advantageous in realizing a high-performance ADC in virtue of its excellent noise-shaping characteristic using over-sampling technique.
Since the sigma-delta ADC has a circuit configuration based on an operational amplifier, it is difficult to be applied to high-speed applications and thus has been limitedly used in only an audio field so far. Through continuing studies on a high-speed sigma-delta modulator, the sigma-delta ADC is being widely used in a wireless communication field as well. The sigma-delta ADC mainly includes a sigma-delta modulator and a digital filter (quantizer). Especially, the sigma-delta modulator is applied to a power amplifier (PA) or a phase lock loop (PLL) circuitry requiring high noise-shaping characteristic.
In general, the sigma-delta modulator may be classified into a lowpass type and a bandpass type, and each type may be divided into a discrete-time structure and a continuous-time structure. Each type has a tradeoff relation therebetween, and is selectively used according to application fields. The discrete-time bandpass sigma-delta modulator may use an external resonator or may be configured based on a switched capacitor resonator in a circuit. The switched capacitor resonator may include a plurality of integrators having an operational amplifier, and integration capacitors and sampling capacitors connected to the operational amplifier through a switch.
Generally, a sigma-delta modulator having a switched capacitor resonator has a limitation in that it is difficult to be used in application fields requiring high-speed performance due to a slow settling time. Therefore, in order for the sigma-delta modulator having the switched capacitor resonator to be applied to a system requiring high-speed performance, it is compulsory to reduce the settling time constant of the integrator based on characteristics of the operational amplifier. Hence, this necessitates the development of a switched capacitor resonator that can reduce the settling time constant to secure high-speed performance of the sigma-delta modulator having the switched capacitor resonator.